This invention relates generally to a semiconductor device, and more particularly to a nonvolatile memory semiconductor device which is capable of storing pieces of analogue information in an exact and nonvolatile way.
In an insulated gate type field-effect transistor having an electric charge-storing means in the insulating layer, a gate threshold voltage of the transistor depends on the polarity and amount of electric charge which is stored in the electric charge-storing mechanism, and therefore the piece of information stored in the form of electric charge can be read in terms of source-to-drain current (or source-to-drain resistance) at a constant gate bias voltage. As a way for storing pieces of analogue information, there can be considered the method of continuously changing the threshold voltage by raising the gate bias voltage within the range of several volts above a given constant value. For performing this, the amount of carrier (electric charge) injected to the electric chargestoring mechanism is continuously controlled.
However, the electric charge stored in the electric charge-storing mechanism bleeds out with time, and therefore if the same amount of electric charge bleeds out, irrespective of the amount of stored electric charge, and if the difference is small between the gate threshold voltage and the gate bias voltage at the time of reading, the "setting" error becomes large, compared with the instance in which the difference is large between the gate threshold voltage and the gate bias voltage at the time of reading, and thus pieces of information are not well stored.